4 ARKIV FOR ZOOLOGI. BAND 7. N:0 '4. 



in the upper jaw: length about 8 mm., thickness about 4 mm.; 

 and in the lower jaw almost similar. 



To show next stage of development I ha ve chosen a young 

 skull of White whale with a basal length of 39 cm. The 

 condition of the teeth of this specimen is rather similar to 

 that of the former. The accessory cusps are somewhat less 

 developed, but this I take to be an individual variation. 

 The chief difference from the smaller specimen lies therein 

 that the teeth are more protniding and that the roots have 

 become a little thitker and very much longer. The length 

 of the roots of the teeth of the upper jaw amounts to 14 — 15 

 mm. and their thickness is about 5 mm. They are cylindri- 

 cal, somewhat curved, and fully open at the basal end. 



These simple facts indicate the general direction of the 

 development of the teeth, and it can easily be followed further 

 on a larger but not too old skull. In such a one with a 

 basal length of 477^ cm. (and only 8 teeth while the two 

 already described have the normal number 9)^ the teeth prove 

 to be worn, the anterior more than the posterior, in a quite 

 conspicuous manner. The small rudimentary crowns of the 

 posterior teeth are, however, not yet worn away but sit like 

 small warts on top of the abruptly thickened roots. In such 

 cases when the crowns are completely worn the tooth is very 

 bluntly conical, except where an upper and a lower tooth 

 have rubbed directly against each other and produced an 

 oblique section of the tooth. The crown of the hindmost 

 tooth of the left side of the maxillary is least worn and 

 shows still very plainly a central main cusp and an additional 

 smaller cusp on either side, in front and behind. The height 

 of this little crown is about 3 mm., and the basal diameter 

 about 2V-2 mm. The visible part of the root which forms a 

 broad basement or footstall for the crown has an antero- 

 posterior diameter of 7Vä mm. 



The hindmost tooth on the right side of the lower jaw 

 has a similar shape, although the anterior additional cusp is 

 somewhat less pronounced. The basal diameter of this httle 

 crown is about 27?. mm. and its height not quite 3 mm. 



^ In some old specimens even 10 teeth may be counted on either side 

 and perhaps some small rudiment behind the tenth but, at least as a rule, 

 only 8 or 9 teeth attain any considerable size so that only those can be 

 regarded as functionary. 



